Metastatic malignancy: This is the most likely diagnosis due to the presence of a hard, painful nodule in a lymph node, which is a common presentation for metastatic cancer. The size of the nodule (19mm) and its location in the femoral lymph node suggest a possible primary malignancy in the lower limb or pelvic region.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy, which may be painful and hard. The femoral lymph node involvement could be part of a systemic disease.
Infectious lymphadenitis: Bacterial, fungal, or mycobacterial infections can cause lymph nodes to become enlarged, hard, and painful. The location in the femoral region might suggest an infection related to the lower limb.
Cat scratch disease: Caused by Bartonella henselae, this condition can lead to painful lymphadenopathy, typically after a cat scratch. The femoral lymph node could be involved if the scratch occurred on the lower limb.

Malignant melanoma metastasis: Given the potential for melanoma to metastasize to lymph nodes, a hard, painful nodule in a femoral lymph node could be a sign of metastatic melanoma, which requires prompt diagnosis and treatment.
Tuberculosis: Although less common in some regions, tuberculosis (TB) can cause lymphadenopathy that is hard and painful. TB is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.

Kikuchi-Fujimoto disease: A rare, self-limiting condition that causes lymphadenopathy, often accompanied by fever and night sweats. It's more common in young women and can mimic lymphoma or tuberculosis.
Kimura disease: A chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia. It's rare and more commonly seen in Asian males.
Castleman disease: A rare disorder that can cause lymphadenopathy, which may be localized or multicentric. It can mimic lymphoma and requires specific diagnostic and therapeutic approaches.